1. Field of Invention
The invention concerns vertically mountable sight gauges for visually determining the level or the volume, or both, of liquid stored in a container. In particular, it concerns such sight gauges as have a substantially transparent pipe surrounded by a rigid, substantially transparent guard or shield.
2. Description of Prior Art
It is known that a sight gauge for visually determining the level of liquid stored in a container may comprise a substantially transparent pipe mounted vertically on the outside of the container, with the pipe being in communication with the container's interior. Usually, the upper end of the transparent pipe in a vertical sight gauge is in direct or indirect communication with the upper interior region of the container, and the lower end of the pipe is in direct or indirect communication with the lower interior region of the container. So arranged, the pressure of the atmosphere above the liquid in the container and that above the column of liquid in the pipe will at all times equalize, and the height of the column of liquid contained in the pipe will indicate the depth of the liquid stored in the container. If the ratio of the internal height of the container to the volume of the container is known, then the level of liquid in the sight gauge readily can be used to calculate the volume of liquid in the container. Indeed, volume graduations may be marked on, etched on, or otherwise directly applied to the gauge.
Often these sight gauges are subjected to considerable stress due to movement or changes in the dimensions of the container. The walls of the container, for example, may bulge during filling. Also, the container wall will inevitably expand or contract with changes in temperature. Tanks can also swell as a result of a build-up in the internal pressure, e.g., due to a rise in temperature of the tank's contents. Changes in atmospheric pressure can also affect the diameter of the container. The greater the size of the container, the greater is the effect of these changes in temperature, pressure, etc. that is transmitted to the sight gauge.
Sight gauges frequently are unable to withstand these stresses, and as a result they often break. Because of their exposure and fragility, they also have been frequently broken by accidental impact. The consequences can be severe. The contents of the container can leak out through the broken gauge, causing a direct economic loss of that commodity, as well as clean-up costs. Serious environmental damage may result as well. If the gauge suddenly ruptures while a worker is reading it or standing nearby, he or she can be seriously injured.
The prior art has attempted to protect sight gauges against impact damage by placing whole or partial shields, or guards, around them. These shields have also served to provide some protection to observers and nearby workers. The shields are not without problems, however.
A type of shield that is commonly used today is comprised of two longitudinal, semicircular halves, which are banded together such that they surround the sight gauge's transparent pipe. The pieces are substantially transparent and rigid and are made of a polymeric resin such as acrylic resin. Each end of the assembled shield is fitted to a housing in which the sight gauge pipe is also mounted. While these shields protect the sight gauge pipes from impact damage, the shields themselves frequently break from the stresses created by deformation of the container walls. Experience has shown it to be a costly and time-consuming process to replace the cracked or broken shields. Replacement is necessary, however, to give the center pipe maximum protection against impact damage--also, to serve as a containment vessel if a center pipe seal should fail or if the center pipe should rupture due to stress or wear. Use of this type of shield requires an expensive inventory of parts. Usually, the length of the sight gauge is approximately equal to the height of the container. Because storage tanks vary in height, the plastic shield halves must be kept in supply in a variety of lengths.
One sight gauge guard described in the patent literature is not as susceptible to stress breakage as the plastic shields just described, but it has other shortcomings. We refer to the sight gauge guard described in U.S. Pat. No. 1,701,914 to Ernst. Ernst discloses a water gauge for boilers. A transparent glass pipe is shielded by two telescoping metal guards. The guards are sprung snugly into housings surrounding the ends of the pipe. Each guard has a semicircular cross-section, so that it does not completely surround the pipe. This permits the observer to read the level of the liquid in the pipe by standing opposite the guards. To facilitate readings, the guards may be rotated to any position. To provide maximum protection against impact damage, the guards are swung around to their outside positions when a reading is not being taken.
One problem with the Ernst guards is that they do not protect the worker when he or she is reading the sight gauge, nor do they provide 360.degree. protection against blows to the transparent center tube. Moreover, the guards will not prevent leakage if the center tube breaks or its seals fail.